In general, a powershift transmission includes a number of gear elements that couple the input and output shafts with a related number of associated clutches that selectively engage the gear elements to move in either the forward direction or reverse direction and establish a desired speed ratio between the input shaft and output shaft. An electronic control system is typically utilized to smoothly engage and disengage a clutch. This electronic control system provides an interface between the clutches through a plurality of associated solenoid valves. The solenoid valves are modulated in order to control the clutch pressures in response to command signals from the electronic control unit.
To precisely time the shifting of the clutches, the fill time becomes an important parameter. Fill time is defined as the time required to fill an oncoming clutch cavity with fluid. During this fill period, the clutch piston will stroke and the clutch plates will move to the point of "touch-up". However, until the clutch plates are compressed together, the clutch cannot transmit any significant torque. Therefore, the end-of-fill time is important to ascertain when this critical moment is reached. A harsh engagement can result in a torque spike that is transmitted through the drivetrain of the machine and creates a "jerk". This jerk is uncomfortable to the operator and diminishes the life expectancy of the associated drivetrain components of the machine.
One known arrangement utilizes a separate flow sensing valve having an electrical switch disposed thereon. The flow into the flow sensing valve is directed through a fixed orifice to the associated hydraulic clutch. Once the flow through the valve ceases, the absence of a pressure drop across the fixed orifice permits the flow sensing valve to return to a spring biased, flow blocking position. Once the flow sensing valve is in the spring biased position, this triggers an electrical switch that indicates that the clutch is filled. A major drawback with this arrangement is that it requires all fluid to flow through a fixed orifice and also through a separate flow sensing switch for each clutch in the system.
Still another known mechanism for determining end-of-fill is to control the amount of time that fluid is allowed to flow toward the clutch. These arrangements do not account for variances in control valves or clutch activating chambers. To overcome these variances, a number of control schemes have been devised to adaptively change the fill time based on previous clutch fills. However, these control schemes depend on costly and time consuming calibration techniques.
Yet another technique for determining the end-of-fill point involves monitoring the electronic activation of the control valve that directs fluid to the clutch. When the actuating chamber of the clutch is full, the increase in pressure operates upon the control valve to move it back to a flow blocking position. The force that is acting to move the control valve back to the flow blocking position is acting against the electrical force that moved the control valve to the flow passing position. This creates an electrical voltage spike that is detected by an electronic controller. This voltage spike represents the end-of-fill point. A drawback with this technique is that a separate control scheme is required for each clutch control valve.
The present invention is directed to overcoming one or more of the problems set forth above.